This invention relates to a multiphase output oscillator, that is, to an oscillator for generating several oscillation signals which are a fraction of a full oscillation period delayed with respect to each other.
An example of an oscillator which generates several oscillation signals is a ring oscillator which contains a cascade of N inverters (N being odd). In operation, the signals at the outputs of the inverters in the ring oscillator will periodically change from high to low and back. The signal at the outputs of successive inverters in the cascade are 180 degrees plus an Nth fraction of a full period of oscillation delayed with respect to each other.
Since N must be odd for the ring oscillator to oscillate, ring oscillators have the disadvantage that they do not directly produce pairs of signals delayed with respect to each other by an even fraction of the period. For example, a ring oscillator does not produce signals which are 90 degrees out of phase. Furthermore, the frequency of oscillation of the oscillator decreases inversely proportional to the number, N, of inverters it contains. The maximum frequency is attained with an oscillator consisting of three inverters. The use of more inverters implies a lower frequency. Therefore, the maximum attainable frequency of a ring oscillator decreases in proportion to the fraction of the full period required as the phase difference.